In working myocardium, acute blockage of blood flow is followed by a rapid drop in oxygen tension that within minutes causes irreversible tissue damage. The onset of ischemic infarction is marked by a cascade of events that at the cellular level includes reduced energy production (-AMP, / ATP, -gycolysis, /pH, -ROS), altered ion channel activity (/ICa, -[K+]o, -[Na+]i), and impaired Ca2+ signaling (/ICa, -diastolic [Ca2+]i) leading eventually to arrhythmia, cardiomyopathy, and heart failure. We hypothesize that the onset of cardiac hypoxia (<60 s) is first detected by a Ca2+ channel regulatory mechanisms leading to rapid channel current suppression long before the global cellular metabolic manifestations (/ATP, /pH, -ROS etc.). To test this hypothesis, we shall perform experiments on single cardiomyocytes exposed to step changes in oxygen tension while ICa and [Ca2+]i are monitored using voltage-clamp and Ca2+-imaging techniques. The changes in pO2 will be implemented with a rapid perfusion system (<50 ms), and will be monitored in the immediate vicinity of the cells. The specific aims are: 1) To characterize the ionic-, voltage-, and phosphorylation- dependence of suppression of ICa in response to acute hypoxia, and 2) To identify the molecular entity that detects the loss of oxygen and the signaling pathway that leads to the modulation of the Ca2+ channel. Significance and Impact: The proposed research might be directly relevant to the management of patients who suffer periods of cardiac hypoxic ischemia. The results may establish hypoxia-induced suppression of ICa as an inherent first line of defense that preserves metabolic energy and delays Ca2+ overload. In a wider perspective, it is important to sort out the various regulatory pathways that are triggered by hypoxia and/or converge to control ICa, force of contraction, and expenditure of ATP. In turn, recognition of the independence or interdependence of these pathways may serve to identify prophylactic and therapeutic options that are relevant to all stages of acute and chronic cardiac hypoxia including e.g. the onset of reperfusion where suppression of ICa is already clinically used to prevent ensuing arrhythmias. If the proposed O2 sensor does indeed contribute significantly to the control of the Ca2+ channel, it may lead to development of new class of therapeutics for treatment of cardiac injury in general. Innovation: It is a novel idea that the suppression of ICa by acute hypoxia can be triggered by a rapid regulatory pathway long before significant occurrence of changes in the cellular energy metabolism, ionic gradients and redox state. To test this idea, we use an array of electrophysiological, optical, and molecular technique that provide simultaneous measurements of key signaling parameters and are suited for kinetic studies. To explore clinical relevance we shall expand the experimental scope from standard animal models to also include available human cardiac cells and cells from the right and left ventricles.